fuzz-commit-graph: initialize repo object

Various #DEFINE "constants" in commit-graph.c now depend on
the_hash_algo->rawsz, but this object must be initialized before it can
be used.

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: steadmon

fuzz-commit-graph.c

@@ -1,4 +1,5 @@
 #include "commit-graph.h"
+#include "repository.h"
 
 struct commit_graph *parse_commit_graph(void *graph_map, int fd,
 					size_t graph_size);
@@ -9,7 +10,9 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
 {
 	struct commit_graph *g;
 
+	initialize_the_repository();
 	g = parse_commit_graph((void *)data, -1, size);
+	repo_clear(the_repository);
 	free(g);
 
 	return 0;